These hormones were shown by the researchers to induce AFP during pregnancy.

They have also been shown to inhibit breast cancer growth in earlier rat studies, although estrogen and progesterone are known to fuel the growth of breast cancer in humans.

Study researcher Herbert Jacobson, PhD, who has been studying AFP in rats for more than two decades, strongly believes the protein is responsible for the pregnancy-related reduction in breastcancer risk.

"Twenty-five years ago I deduced that this must be the agent responsible for lowering breastcancer risk in women who have been pregnant," he tells WebMD. "And the research we have done since then supports this hypothesis."

AFP and Breast Cancer

Pregnancy, especially before the age of 30, is known to lower a woman’s lifetime risk for developing breast cancer, and having more than one child is also protective.

Alpha-fetoprotein is made by the fetus, and measurement of the protein during pregnancy can help screen for birth defects.

Very high AFP levels, for example, suggest the presence of neural tube defects or an abdominal wall defect known as omphalocele, and very low levels are suggestive of Down syndrome.

The protein is usually undetectable in the blood of healthy men and healthy women who aren’t pregnant. In these groups, elevated AFP levels suggest the presence of certain cancers.

In their new study, which appears in the December issue of Cancer Prevention Research, Jacobson and colleagues treated cancer-exposed rats that were not pregnant with estrogen, estrogen plus progesterone, or human chorionic gonadotropin (HCG).

As has been seen in previous studies, all three treatments were associated with a reduction in breast cancers in the high-risk rats.

All three of the hormone treatments were also associated with elevated AFP levels and AFP was found to directly inhibit the growth of breast cancer cells grown in lab cultures.

"Hormones in pregnancy, such as estrogen, all induce AFP, which directly inhibits the growth of breast cancer," Jacobson says in a news release.